1. Field of the Invention
This invention relates to apparatus for measuring the electrical impedance of materials, and in particular relates to apparatus for measuring the electrical impedance of a given volume or area of sample material and relating this impedance to a particular characteristic of the material such as moisture content.
2. Description of the Prior Art
Optimization of the yield of various manufacturing processes often requires very accurate monitoring of the moisture content of a given material. For example, the milling of wheat is carried on most efficiently when the wheat has a 15% moisture content. During the pulping of wood chips, the moisture content of the wood chips must be known in order to determine the proper amounts of liquor necessary to be added for maximum delignification. The required accuracy for the measurement of moisture varies for different materials and is shown below for some representative materials:
______________________________________ Required Accuracy Moisture of Moisture and Percent of Material Range Indication Full Scale ______________________________________ Wood Chips 30%-70% .+-. 3% .+-. 7% Wheat 8%-15% .+-. 0.2% .+-. 1.4% Paper 5%-10% .+-. 0.5% .+-. 5% Plywood Veneer 3%-8% .+-. 0.5% .+-. 6% Potato Chips 12%-17% .+-. 0.5% .+-. 3% ______________________________________
The most exacting measurement requires an accuracy of 1.4% of full scale.
Known moisture measuring systems for the above listed materials have, in general, proved unreliable for a variety of reasons including the instability of the bridge circuitry used to measure the electrical impedance of samples and the failure of the known systems to compensate for changes in temperature and density of the material, both of which critically affect the measurement of impedance. While the impedance reading may be compensated for these variations under laboratory conditions, no practical system is known for making the necessary corrections while measuring impedance "on-line" as is required for continuous process control.
Known moisture detection apparatus include those described in U.S. Pat. No. 3,391,337 and U.S. Pat. No. 3,430,140, which patents list applicant as inventor.
The degree of accuracy necessary for accurate moisture content measurement may be illustrated by noting that to determine the moisture content of wheat having a moisture content of beween 8% and 15% with an accuracy of .+-. 0.2%, i.e., an absolute accuracy of about 1.5% of the full scale measurement, all critical components of the moisture meter must remain constant to within about .+-. 0.1% so as not to contribute a total accumulated error of more than one half of the desired accuracy. A 25 cubic inch sample of wheat having a 10% moisture content has an impedance with an ohmic component of about 100,000 ohms and a capacitive component of about 30 picofarads. A small change in moisture content may only be measured accurately if the impedance bridge components of the moisture content measuring meter are constant with respect to thermal drift and aging to an accuracy of at least 0.01 picofarads.
Temperature variations may also considerably influence the impedance of a sample material. Impedance of wood chips, for example, changes by a factor of 2 for a 40.degree. C. change above the freezing point. In addition, a discontinuity in impedance exists at the freezing point and the rate of change in impedance below the freezing point is drastically different. Consequently, a meter which is effective for measuring the moisture content of a material must have a built-in temperature correction capability capable of correcting for aberrations in impedance existing over a wide temperature range.
In measuring the moisture content of bulk materials, density is another critical factor. For a bulk sample having a constant moisture content, impedance varies inversely with density. One possible way of correcting moisture measurement for variations in density involves the measuring of samples of constant weight. It has been found, however, that the moisture reading of a sample is not exactly proportional to the inverse bulk density of the sample and thus this method requires the use of additional calibration charts in order to produce a usable reading. A second technique for compensating for variations in density involves constant volume sampling, and this technique has been found to be more reproducible and versatile in that the weight of a sample of constant volume may be directly measured and introduced as a corrective factor in the ultimate moisture content measurement.